[Vanilla] How are we creating Unit Icons these days?

From what I've see in other mods, it looks like simply a black square with their unit art icon affixed.

Does the game define the circle of the icon, and all we need to do is create an alpha layer in the shape of our icon for the icon to show through?

I recall in civ 5 we had to we had to have a circular alpha with everything inside the circle our colored unit icon...

Romans 10:9-10 “That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.”
Acts 4:12 “Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.”